Zheng et al. Microsystems & Nanoengineering (2020) 6:89 Microsystems & Nanoengineering https://doi.org/10.1038/s41378-020-00199-x www.nature.com/micronano

ARTICLE Open Access -based fabrication of microscale 3D structures Fengyi Zheng 1, Zhongyan Wang1,JiashengHuang1 and Zhihong Li1

Abstract This study proposed a method for fabricating 3D microstructures of ice without a supporting material. The inkjet printing process was performed in a low humidity environment to precisely control the growth direction of the ice . In the printing process, droplets (volume = hundreds of picoliters) were deposited onto the previously formed ice structure, after which they immediately froze. Different 3D structures (maximum height = 2000 µm) could be formed by controlling the substrate temperature, ejection frequency and droplet size. The growth direction was dependent on the landing point of the droplet on the previously formed ice structure; thus, 3D structures could be created with high degrees of freedom.

Introduction phenomenon11,12, ice spike phenomenon13, ice layer In the past decade, inkjet technology has become spreading along a substrate14 and freezing of droplets15,16 recognized in the technical community as a highly capable and bubbles17. Ice has also been widely used in micro- tool for manufacturing, particularly micromanufactur- fabrication, such as ice templates18 for biomaterial sys- 1,2 19–21

1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; ing . In micromanufacturing, inkjet deposition processes tems, ice lithography for nanopatterning methods are used to produce a pattern of material on a substrate. and ice blocks for microfluidic channels created by laser Most micromanufacturing applications of inkjet technol- processing22. However, these technologies are based on ogy deposit phase change liquid inks onto nonporous removal techniques and require expensive equipment and substrates, which solidify quickly after deposition. Exam- an additional supporting layer or protection layer. ples of phase change materials in micromanufacturing Utilizing water as a phase change material in 3D applications include solders for electronic manufactur- printing, we proposed an economical additive manu- ing3,4 and thermoplastics for free-form fabrication5,6. The facturing method to fabricate ice structures by inkjet control of spreading by solidification is a beneficial aspect printing, which is called “ice printing”23. A small liquid of phase change materials for applications where the goal droplet can be printed separately from the nozzle onto a is to limit spreading and obtain the smallest spot for a cold surface, after which the liquid droplet immediately given droplet size7. freezes into an ice . Then, an increasing number of Water is the most common substance in nature and has water droplets can be printed and interconnected to substantial advantages in free contamination. The phase create a specific structural design. This ice structure can transition from water to ice is a phenomenon that then be used in various applications under the ice point. often occurs8. Studies have been applied to determine However, in its current form, this ice printing method is the macroscale mechanism and process of icing, such limited to simple planar structures close to the substrate, as freezing front propagation9,10, supercooling such as microcapsule ice arrays for reagent presealing24,25 and parallel ice channels for microfluidics26. In this paper, we optimized the process parameters to precisely control Correspondence: Zhihong Li ([email protected]) the growth direction of , fully realizing the 1National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Institute of Microelectronics, Peking University, Beijing 100871, concept of 3D ice printing. A variety of ice structures with China

© The Author(s) 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a linktotheCreativeCommons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Zheng et al. Microsystems & Nanoengineering (2020) 6:89 Page 2 of 10

different geometries, sizes, heights and overhanging quickly moves to the next voxel in the same layer. The structures were successfully produced. In contrast to vertical distance the nozzle travels between subsequent other 3D printing methods, such as fused deposition layers is held constant throughout the printing process. modeling and stereolithography, the ice printing method The voxel is considered printed when the droplet freezes. does not need additional supporting structures or removal The distance from the top of the ice structure to the processes, which simplifies the geometric design and bottom of the nozzle is fixed at 2 mm. Free-standing avoids the unnecessary introduction of other chemicals. structures and overhanging angles are feasible without Moreover, the 3D ice printing method does not require supporting structures. specific substrates. The ice structure can be formed on any theoretical substrate that is sufficiently cold for icing, Effect of the experimental environment such as glass, metal, silicon, and polymer films. Further- The icing process is mainly affected by the type of more, 3D-printed ice structures can be used as soft solution, the temperature field above the substrate, the lithography molds for complex microfluidic channels, environmental humidity and the printing frequency. which is difficult or even infeasible for conventional Glass substrates are used in the following discussion. photolithography methods. In addition, 3D ice printing The height of the printed features had a variation of has the potential to create 3D porous scaffolds of metal 1–3.5%, which indicates the reproducibility of the ejected salt , which precipitate from the water dur- droplet volume. Therefore, inkjet printing was found to be ing the freezing process and then occupy the spaces a suitable patterning technique for highly reproducible between the ice grains. processes.

Results Solution Printing process The formation of ice crystals is mainly influenced by the A customized inkjet printing system has been made to type of solution. Here, we divide the solutions into two demonstrate the 3D ice printing method. Figure 1a shows categories. For ultrapure water, the supercooling phe- a schematic view of this system, which consists of a pie- nomenon severely interferes with the formation of ice zoelectric nozzle, a three-axis movement platform and a structures on cold substrates. As a small volume droplet is self-built cooling subsystem. The ice printing process is free of impurities and bubbles, even if the temperature is voxel based. Voxels are defined as elementary 3D blocks. below 0 °C, it will remain liquid instead of freezing27. The A single voxel is built from the phase change of a droplet supercooling phenomenon only occurs when droplets from liquid (water) to (ice). The size of the voxel is contact a clean substrate. When the droplets contact the related to the generated droplet, which depends on the printed ice structure, they always freeze. The supercooling nozzle size and the applied pulse voltage signal. Stacked in phenomenon is a probabilistic event in the ice printing a layer-by-layer sequence at defined coordinates, the process because the droplets have a certain initial velocity voxels form the desired 2D or 3D geometry. Figure 1b when they hit the substrate, and the nonsteady state will demonstrates the basic working principle of ice printing. help the droplets freeze. To reduce the probability of the The solution is stored in a reservoir and is driven by the supercooling phenomenon, an effective method is to air pressure to fully fill the capillary in the piezoelectric minimize the temperature of the substrate. Related stu- nozzle. During the printing process, the droplets colliding dies show that the crystallization rate of water reaches a with the ice structure spread out and freeze immediately maximum at approximately −48.15 °C, which implies the via thermal conduction without a rebounding motion. lower limit of metastability of liquid water28. Thanks to Controlled by a computer, the modified inkjet printer can the two layers of Peltier coolers in the ice printing system, print droplets of different sizes at different frequencies in the current ice printing system can reach −56 °C. Another desired locations during the manufacturing process, and method is to modify the surface of the substrate, either frozen layers are repeatedly formed with a controllable introducing irregular structures to disrupt the steady state thickness and diameter, leaving ice patterns on the sub- of the droplet or introducing particles to help form the ice strate. Theoretically, this ice printing method is suitable core. For water containing impurities, its freezing point for a variety of substrates with good thermal conductivity. will change, which can be calculated using the following The ice printing system can directly print 3D ice formula structures with micron resolution by additive manu- facturing. The ice structures are flexible with respect to à nB δT ¼ Tf À Tf ¼ Kf bB ¼ Kf ð1Þ size, geometry and height. Figure 1c–e demonstrates the mA layer-by-layer printing process. The word “ICE” contains When printing multiple solutions with different freezing vertical structures, curved structures, and overhanging points, selective removal of the ice structure can be structures. After a single droplet is printed, the nozzle achieved by adjusting the temperature of the substrate. Zheng et al. Microsystems & Nanoengineering (2020) 6:89 Page 3 of 10

a b Air pressure

Air pressure provider Three-axis movement platform Solution reservoir

Voltage excitation signal Piezoelectric nozzle

Circulating water

Piezoelectric nozzle Nitrogen atmosphere CCD cameras Strobe light (%RH < 0.1%) Voxel growth

Copper substrate Peltier coolers Radiating plate DC Peltier cooler

cde

Designed structure Layer-by-layer sequence Pillar Curve 90° overhang angles

Voxel growth Second layer First layer substrate Copper substrate Copper substrate Peltier cooler Peltier cooler Peltier cooler

Fig. 1 Schematic view of the principle of ice printing process. a Schematic view of the ice printing setup. b Operating principle of ice printing technology. The temperature of the substrate is controlled by Peltier coolers, which are powered with a direct current (DC) voltage source. The droplets are provided locally from the piezoelectric nozzles, thereby confining the freezing process for local ice structure patterning. The size of the droplets is controlled by the air pressure and the voltage pulse signal. c–e Layer-by-layer printing process of a 3D structure

Temperature temperature of the substrate was set to −30 °C, the height The growth process of the ice crystal structure is mainly of the cofferdams on the substrate was set to 2, 4, and influenced by the temperature field around the substrate. 8 mm. The simulation results shown in Fig. 2c reveal that Experiments have shown that ice structures should be a higher cofferdam helped increase the −4 °C isotherm produced at a maximum temperature of −4 °C; otherwise, height. the droplet freezes too slowly, destroying the intended In addition, the nozzle also influenced the tempera- structural design. The −4 °C isotherm height is the first ture fieldarounditself,asshowninFig.2d. First, the reason limiting the height of the ice structure. Compared nozzle should not be too close to the ice structure. to the isolation container housing the entire printing The distance between the bottom of the nozzle and the system, the cooling system was very small. Therefore, we top of the ice structure was defined as d. The minimum built a heat transfer model in an environment with an value of d that still allowed the droplet to freeze stably was unlimited nitrogen supply. The temperature field above defined as dmin. As the ice height increases, the tem- the substrate simulated by COMSOL Multiphysics is perature of the ice surface may also increase. To reduce presented in Fig. 2a. The temperature of the bottom of the the thermal effect from the nozzle, the nozzle should be substrate was set to −10 °C to −30 °C at 5 °C intervals. As far from the ice, which means that dmin increases as the shown in Fig. 2b, as the temperature of the substrate height of the ice structure increases. Second, the nozzle decreased, the −4 °C isotherm height increased sub- should not be too far from the ice structure. Because of stantially. Cofferdams made from good thermal con- the temperature gradient, the convection from the air is ductors, such as copper, can maintain the same strong above the substrate, and a long distance will cause temperature as the Peltier coolers and help control the the flying droplet to have a random motion and deviate temperature field above the substrate. When the from the intended position. Considering the accuracy of Zheng et al. Microsystems & Nanoengineering (2020) 6:89 Page 4 of 10

ab4 c4 mm 2 2 160 0 0 150 19 17 140 –2 –2 15 130 –4 –4 13 120 11 –6 –6 110 9 –8 –8 100 7 –10 –10 90 5 –12 –12 80 3 1 –14 –14 70 Temperature of the substrate Height of the cofferdam –1 –16 –16 60 –3 50 –18 –10 °C –18 2 mm Temeperature (°C) –5 –20 Temeperature (°C) 40 –7 –15 °C –20 4 mm 30 –9 –22 –20 °C –22 6 mm 20 –11 –24 –25 °C –24 10 –13 –26 –26 0 –15 –30 °C –17 –28 –28 –10 –19 –30 –30 0 50 100 150 mm 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Height (mm) Height (mm) d e d1

20 d2 100 mm When 15 d > 0 d2, the droplet cannot fly stably

–100 10 ) d –200 150 5 Suitable interval of d for ice printing process 100 0 )

50 max h –5 0 d1, the droplet cannot freeze d < –10

and the ice structure ( When 100 Distance between the nozzle –15

z 0 ice structure (

y x mm Maximum height of the –100 –20 Height of the ice structure (h) Fig. 2 Thermodynamic simulation of the manufacturing environment in COMSOL Multiphysics; note that the red lines correspond to −4 °C. a Thermal field distribution of the cross section at the center of the ice structure. b Temperature change with respect to the height along the center of the structure for substrate temperatures ranging from −30 °C to −10 °C. c Temperature change with respect to the height along the center of the structure for different cofferdam heights (2, 4, and 8 mm) when the substrate temperature is −30 °C. d Effect of the nozzle on the thermal field distribution during printing. e Intervals of the distance between the nozzle and the ice structure for ice structures with different heights

the deposition on the substrate, a shorter distance surface, which will keep the substrate clean and the sur- between the nozzle and substrate will subsequently face of the ice structure smooth. Otherwise, the water reduce the disturbance of the airflow on the flying droplet. molecules in the environment will form ice crystals on The maximum value of d that still allowed the droplet to their surfaces. Usually, the humidity is set below 0.01% fly stably was defined as dmax. Figure 2a shows that the RH. To control humidity, the whole system was placed isotherm became sparser as the height of the structure inside a container to keep it isolated from the outside. increased. In this case, the temperature changed faster Pure nitrogen, controlled by a flow meter, was passed into and the thermal convection became more intense, which the container to adjust the humidity. Although evapora- had a greater impact on the flying droplets. Therefore, tion and sublimation always exist in the printing and dmax decreased as the height of the ice structure increased. freezing process, this setup ensured that the water For a stable fabrication process, d should always be within molecules diffusing into the environment had less effect the bounds of dmin and dmax. The available intervals of d on humidity. In the case of continuous printing, an for ice structures with different heights is the second ultralow humidity environment can exist for at least reason limiting the height of the ice structure. In accor- four hours. dance with the simulation results and previous experi- mental experience, when the height of the ice structure Printing frequency was less than 2 mm, the distance from the nozzle to the The morphology of a frozen droplet is greatly affected top of the ice structure was set to 1 mm. by the printing frequency. In the discussion in this sec- tion, the nozzle did not exhibit displacement in the x-y Humidity plane. Hence, all of the droplets fell on the same point. The preservation of an ice crystal structure is mainly The substrate was a glass slide, and the temperature of the influenced by humidity. In a stable low-temperature substrate was −27 °C. environment, the main factor affecting the stability of When the printing frequency was lower than 20 Hz, the ice is sublimation, especially for microstructures fabri- freezing of each droplet was independent. As the volume cated by ice printing. The sublimation rate increases of the droplet was only hundreds of picoliters, the droplet substantially with decreasing humidity. However, a dry had no impact or retraction on the substrate, and it froze atmosphere is necessary to avoid frosting on the cold in less than 50 ms, which was the main difference from Zheng et al. Microsystems & Nanoengineering (2020) 6:89 Page 5 of 10

ab cdefg

Droplet Droplet Droplet

Protrusion on the top The diameter of the ice structure at the current height Larger Frozen droplet Current large “substrate” droplet Ice Ice Ice

Substrate (–27 °C) Substrate (–27 °C) Substrate (–27 °C) Substrate (–27 °C)

Fig. 3 Ice torch with a large top and a small base. a–d Schematic view of the formation process. a The droplet freezes quickly on the cold substrate. b The ice structure acts as a new substrate. Droplets on the ice structure gather into a larger droplet before freezing. c The larger droplet freezes. The diameter of the ice structure increases as its height increases. d The ice torch also has a protrusion on its top. e–g Influence of printing frequency on freezing structure. The shape of the ice torch depends on the total liquid volume and the printing frequency. The printing frequencies are (e) 500 Hz, (f) 200 Hz and (g) 50 Hz. All scale bars are 300 μm

the macroscale behavior of the droplet29. Each droplet When the printing frequency is higher than 1500 Hz, was a smooth sphere in the liquid state. When the droplet the droplets are more likely to gather into a large droplet. froze, because the freezing process occurred from the The liquid droplets will completely cover the previously bottom up30 and the water swelled during the freezing formed ice structure. In the ice printing process, the process, the ice bead had a protrusion on the top. printing frequency is usually less than 1000 Hz. Although this phenomenon was inevitable, the angle of Furthermore, we observed faster vertical growth in the the protrusion was not affected by the rate of solidification “trigger” mode. The trigger mode is a manual operation but depended on the water density-to-ice density ratio31, mode in which the printing frequency f is very high the wetting angle32 and the substrate temperature33. For (usually greater than 1000 Hz). Different from the con- droplets of the same volume, the ice beads were always tinuous printing mode, in the trigger mode, the number of conical, so this phenomenon did not affect the resolution droplets N instead of the printing time T is set for each of ice printing technology but only limited the smooth- printing action. When N is far less than f, it seems that ness of microscale ice structures. the printing action only produces one “large droplet” on When the printing frequency was higher than 20 Hz and the previous ice structure. Under this circumstance, the less than ~1250 Hz, the printing created torch-shaped ice printing action is called a trigger. This result allows a structures with large tops and small bases, which was dynamic scaling of the ice printing technique, where the influenced by the time of the continuous printing (Videos size of droplets in each trigger is adjusted according to the 1–3, Supporting Information). The related results are desired feature size: depending on the size of the nozzle listed in Fig. 3. Because the substrate was very cold, (20–80 μm) and the number of droplets in each trigger droplets that contacted the substrate quickly froze. For (1–20), we could produce voxels from 30 to 200 μm droplets that were deposited later, the existing ice struc- in width. ture acted as a new platform. The ice structure was not as cold as the substrate, which caused the droplets to freeze Growth of the ice structure at a slightly slower rate and provided time for the droplets The growth properties of the ice crystals deposited on to accumulate in the liquid state prior to freezing. an ice structure were recorded using a CMOS camera According to the abovementioned simulation results of with a strobe light (Video 4, Supporting Information). The the thermal field, the temperature around the top of the ejected droplets of ultrapure water had a constant volume ice structure increased as the height of the ice structure (~150 pL). The frequency of the droplet ejection was increased. Therefore, the freezing rate of the droplets approximately 1 Hz. The height change of the ice struc- became slower, and the time required for the droplets to ture from a single droplet is demonstrated by a set of accumulate increased. Under these circumstances, the strobe photographs from collision (0 ms) to freezing diameter of the ice structure increased with respect to the (300 ms) in Fig. 4a. The fitting curve in Fig. 4b shows that height of the structure, which led to a torch-shaped the printing process had a relatively constant layer structure. Because the number of droplets on the ice thickness per droplet (28 ± 3 μm). As the pillar was structure at a certain height before freezing increased with formed by a sequence of droplet layers, the outer sidewall respect to the printing frequency, the slope of the side was was not smooth, which is inevitable in the ice printing positively correlated with the printing frequency. process. Moreover, because the ice crystal on the top of Zheng et al. Microsystems & Nanoengineering (2020) 6:89 Page 6 of 10

abcd

0 ms 45 ms 90 ms 135 ms 180 ms 225 ms 1500 R 2 = 0.996 k = 28.885 m) µ

1000

Structure height ( 500

0 0 5 10 15 20 25 30 35 40 45 50 55 Number of droplets Fig. 4 Straight ice structures fabricated by ice printing. a Time-sequence images showing the behavior of a single droplet deposited on a microscale ice structure; note that the time interval is 45 ms. b The height of the structure increased in proportion to the number of droplets. c A completed ice pillar and an ice needle. d An array of ice pillars. The heights of the ice pillars in the front line, middle line, and back line are 5 voxels, 10 voxels and 20 voxels, respectively. The scale bars are 150 μm

the pillar gradually sublimated in the ultralow humidity Printing results environment, the straight ice pillar gradually turned into a Figure S1 illustrates the printing process and the needle, as shown in Fig. 4c. Based on this phenomenon, printing results of flat graphics. Arbitrary patterns were although there were some restrictions on spatial degrees created by exploiting the x-y positioning capabilities of the of freedom, structures smaller than the size of a single movement platform (Video 5, Supporting Information). droplet could also be fabricated. Figure 4d shows an array A range of 3D structures is displayed in Fig. 6. Figure 6a of pillars with different heights. is a helix on one voxel in diameter; the voxels are stacked Pillars with different slopes can be achieved by con- in one dimension. Figure 6b is a wall structure with dif- trolling the horizontal space between printed droplets, as ferent heights from 5 voxels to 25 voxels and a wall illustrated in Fig. 5a. When printing automatically, the thickness of 1 voxel; the voxels are stacked in 2 dimen- horizontal space l is related to the ratio of the nozzle sions. Figure 6c is a pyramid where the voxels have been velocity v and the printing frequency f as follows: stacked in 3 dimensions. Figure 6d shows a combination of four connecting walls and a pillar in its center, which v – l ¼ ð2Þ were all printed in one run. Figure 6e g shows a stickman, f a three-story ladder and a Chinese character (zheng). A In most instances, l should be smaller than the radius of microscale ice alphabet is shown in Fig. 6f. The repro- the frozen droplet; otherwise, the ice pillar will be ducibility of the printing structure is shown in Fig. S3 discontinuous rather than the designed shape. In Fig. (Supporting Information). 5b, the velocity of the nozzle is fixed at 6 mm/s, and the printing frequencies—from left to right—are 125, 100, 75, Discussion and 50 Hz. Note that l affected both the layer height h Utilizing the self-built printing system, precise control (Fig. 5c) and the curve angle θ (Fig. 5d). of various microscale ice structures was achieved. How- In addition, overhanging structures with 90° angles or ever, as icing is a complicated physical process, the shape even negative angles can be fabricated by manual control. of the frozen droplet and its position on the ice structure In the manual control mode, the landing point of a dro- are not always as expected. There are still few limitations plet is predicted through the charge-coupled device in ice printing technology. (CCD) camera. When printing each droplet, we managed One possible limitation of ice printing is the growth to make the droplets, which flew obliquely downward, characteristics of ice crystals. Droplets always freeze contact the side of the ice structure. In this way, the fromwheretheicecorefirst forms. As previously droplet will continue moving downward along the surface mentioned, the ice voxel is an irregular hemisphere of the ice structure instead of staying steadily on the top of with a protrusion on its top. The protrusion provides the ice structure. Because the volume of the droplet is disturbance to the droplet, which increases the prob- very small, it will freeze and become an extension of ability of the ice cores forming here first. When local the existing ice structure before falling off of the ice growth is initiated, the generated protrusion will loca- structure. In this way, horizontal growth and diagonal lize the growth of the ice structure. Although the wall in downward growth of the ice structure are achieved. Figure Fig. 6b was printed horizontally in a sequence of lines, 5e demonstrates the growth process of a horizontal the wall contains grooves and appears as vertically structure. aligned pillars. Zheng et al. Microsystems & Nanoengineering (2020) 6:89 Page 7 of 10

a c

35 Droplet 30 m)

µ 25

l h 20 Collision point 15

10 T  FREEZE 5 Layer height per droplet ( 0

Ice structure Ice structure –5 –10 0 10 20 30 40 50 60 70 Horizontal space (µm) Horizontal space l Layer thickness T Layer height h Curve angle 

b d 00 90 500 µm 500 µm 500 µm 500 µm 80 70 60 50 40

Curve angle 30 20 10 0 10 –10 0 10 20 30 40 50 60 70 Horizontal space (µm) e

500 µm 500 µm 500 µm

Fig. 5 Curve structures fabricated by ice printing. a Mathematical model of the geometric design to estimate the relationship among the curve angle of the structure (θ), the horizontal distance (l), the layer thickness (T) and the layer height (h). b Ice pillars with different slopes printed when θ = 0, T = h, and l = 24 mm, 30 mm, 40 mm, and 80 mm. c Relationship between l and h. d Relationship between l and q. e The fabrication process of a horizontal beam

Another possible limitation is that the voxel-based fabri- A third possible limitation is the instability of the icing cation method makes the outer surface of the ice structure process, especially when printing structures with nearly rough, especially when machining bevel structures. A close 90° angles or even negative angles. Taking the ladder in inspection of the pyramid in Fig. 6c reveals an obviously Fig. 6f as an example, the three horizontal lines were all stepped bevel. For printing results without overhanging printed from left to right, but the final shapes are not structures, we can slowly increase the substrate temperature exactly the same. Assuming that the printing state of the to −4 °C, and there will be a quasi-liquid layer on the nozzle is stable, the air flow caused by the uneven tem- solid–vapor interface of the ice structure. The thin water perature field will affect the flight path and the subsequent film adheres tightly onto the ice structure and will by landing point of the droplet. When the droplets flew smoothed by its . When the ice structure obliquely at different angles and hit the side of the pre- freezes again at a low temperature, the thin water film will vious structure, the ice structures had different protrusion form a relatively smooth outer surface. orientations, thereby affecting the next icing process. Zheng et al. Microsystems & Nanoengineering (2020) 6:89 Page 8 of 10

a bc

1 mm 1 mm 1 mm

d efg

1 mm 1 mm 1 mm 1 mm

h

1 mm Fig. 6 Images of 3D-printed ice structures at the microscale. a A single helical ice structure. b An ice wall with various heights. c An ice pyramid. d A composite ice structure. e An ice stickman. f An ice ladder. g A Chinese character (zheng). h An ice alphabet including various 3D overhanging structures without support

Conclusion excitation signals, the spot sizes can be tuned. In addition, In conclusion, we have demonstrated ice printing tech- as nanowires have been successfully assembled in macro- nology as a 3D fabrication method, which represents a new scale ice structures34,35, microscale ice structures can step in the direction of using inkjet printing technology for potentially support 3D porous scaffolds for materials that the microfabrication of ice structures. Ice patterns with are soluble or dispersed in a liquid of metal salt nano- defined geometries were created with a resolution in the particles, which precipitate from the water during the range of tens of micrometers by providing the desired freezing process and then occupy the spaces between ice droplets on the cold substrate. Using different voltage grains. We plan to explore the suitability of our setup to Zheng et al. Microsystems & Nanoengineering (2020) 6:89 Page 9 of 10

enable the fabrication and preservation of complex three- The whole ice printing system was placed in a container dimensional ice structures with different materials. Fur- with a nitrogen atmosphere to isolate the system from the thermore, the produced microscale ice structures can be outer environment. Icing is a complex physical process used as a mold for complex microfluidics with presealed during which humidity and temperature are the main liquids, which is difficult or even infeasible for conven- factors. Hence, a digital thermometer and hygrometer tional photolithography methods. In the future, this pro- (Sensirion, Switzerland) were used to monitor the envir- tocol can potentially enable biomicrofluidic devices, such onment inside the container. as drug mixing and delivery systems. The system is flexible, enabling the user to design cus- tom deposition processes. For example, here, we added Methods two CCD cameras (front and side) to enable visualization Printing system of the nozzle loading, the generated droplets and the A custom inkjet printing system was constructed to printed structures. demonstrate the 3D ice printing method. Figure S2 shows a When manually controlled, the printing process was photograph of the system, which consists of a piezoelectric monitored by the two CCD cameras. The landing point of nozzle, a three-axis movement platform and a self-built the droplet can be confirmed by observing the position on cooling subsystem. The Piezoelectric nozzles, air pressure the screen. When the printing process was automated by a controller and voltage signal generator were purchased from preset program, the trajectory, the movement speed and MicroFab Technology, USA. The inside diameters of the the voltage control of the nozzle were specified in a.tsp piezoelectric nozzles ranged from 2 to 60 μm, which could file. The file was loaded into the printer software. The.tsp meet the printing needs of solutions with different files were generated by a design assistant called TSAPS, and compositions. The ranges of the three-axis movement which was provided by Shenteng Technology, China. platform in the x, y and z axes were 300, 150 and 40 mm, Alternatively, the files can be generated by any third-party respectively. All three stages had a 10 μm resolution, which software capable of exporting CAD files. satisfied the requirements of microfabrication. The platform Acknowledgements could be manually controlled with a handle or automatically This work was supported by the National Key Research and Development controlled with a computer program. The signal coupling of Program of China (No. 2016YFA0200802) and the 111 Project (B18001). the piezoelectric nozzle and the movement platform was achieved through an I/O controller (ZLAN 6802) connected Author contributions F.Z. conceived the experiments; F.Z., Z.W., and J.H. conducted the experiment with a computer. (s); and F.Z., Z.W., and Z.L. analyzed the results. All authors reviewed the A cold surface was required for the working platform on manuscript. which freezing occurs. The cooling system (which could − Conflict of interest reach a minimum temperature of 56 °C) consisted of The authors declare that they have no conflict of interest. two Peltier coolers and a radiating plate with circulating water at 5 °C. The DC voltage source controlled the heat Supplementary information accompanies this paper at https://doi.org/ transfer from the cold face to the hot face of the Peltier 10.1038/s41378-020-00199-x. coolers. The radiating plate, which was connected to a Received: 21 April 2020 Revised: 30 June 2020 Accepted: 19 July 2020 water-cooling system, was used to release the heat from the hot side of the Peltier coolers. These two parts had a uniform layer of -containing thermal grease between them to enhance heat transfer. A platinum thermocouple References was used to measure the working temperature of the cold 1. Singh,M.,Haverinen,H.M.,Dhagat,P.&Jabbour,G.E.Inkjetprinting—process surface in the cooling system. All the self-built cooling and its applications. Adv. Mater. 22,673–685 (2010). 2. Kim, J. Y. et al. Inkjet-printed multicolor arrays of highly luminescent systems were surrounded by foam insulation to isolate the nanocrystal-based nanocomposites. Small 5,1051–1057 (2009). heat exchange with the environment. 3. Hayes, D. J. Picoliter solder droplet dispensing. Int. J. Microcircuits Electron. During the printing process, the droplets collided with Packag 16,173–180 (1993). 4. 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